Update documentation tooling recommendations

[matt-graham]

@paddyroddy and I think we should maybe switch to recommending both Mkdocs and Sphinx as 🟢 as they seem to offer similar level of functionality / extensions and both are mature / well used.

[p-j-smith]

Yeah I agree, I used Mkdocs for the first time recently and really like it. I would probably even suggest using it over Sphinx if someone is not familiar with either of them

[paddyroddy]

Yeah, I would actually play MkDocs way ahead of sphinx. Most people's views on sphinx come from the fact that it is much more mature, and comparisons are largely from when MkDocs was in its infancy. I think we should make our cookie docs in #115 in MkDocs personally.

[matt-graham]

I don't have strong opinions favouring Sphinx, but I think much more mature is a valid reason to favour one tool over another for a default recommendation. Sphinx has more regular contributors than MkDocs and is more widely used, so I'd say it's less at risk of becoming non-maintained. For generating API documentation autodoc is a built-in extension in Sphinx, while mkdocstrings is a relatively young project and most of development seems to be by one main contributor. There is some interesting related discussion here. I'd personally err towards recommending both with an explanation of something like

Sphinx is the de-facto standard that is widely used. It is well tested, reliable and very customisable.
Mkdocs is a more recent but still relatively mature documentation system that can be simpler to set up than Sphinx.